The present invention relates to an apparatus for improving the surface quality of rotary machine parts, which apparatus employs as a heat source a high density beam such as a laser beam or electron beam and which is capable of providing a solidified layer formed by rapid solidification of a molten surface portion of a machine part and a transformation-hardened layer formed below the solidified layer without cracking or pitting.
FIG. 1 shows schematically a conventional surface improving apparatus for rotary machine parts, in which reference numeral 1 depicts a high density beam generator, 2 a high density beam obtained therefrom, 3 a condenser for condensing the beam, 4 a rotary machine part whose surface is to be improved, and 5 a positioning mechanism for positioning the rotary machine part 4 while rotating the latter.
The high density beam 2 from the beam generator 1 is condensed by the condenser 3 and directed onto a surface of the rotary machine part 4, which is rotated at a constant speed while being held at a designated position by the positioning mechanism 5.
FIG. 2 is a perspective view illustrating a spatial intensity distribution of the high density beam 2 condensed by the condenser 3 and directed onto the surface of the machine part 4. As is clear from FIG. 2, the spatial intensity distribution of the beam is substantially conical, the higher the intensity, the closer to the center of the beam.
FIG. 3 is a partially cross-sectioned side view of the rotary machine part 4, which may be a cam shaft whose cross section is noncircular and whose surface quality has been improved by the conventional apparatus shown in FIG. 1, and FIG. 4 is cross-section taken along a line IV--IV in FIG. 3. In these drawings, reference numeral 6 depicts solidified layers formed by rapidly solidified surface portions of the cam shaft 4 which are rendered molten by irradiation with the high density beam 2, 7 transformation-hardened layers formed below the solidified layers 6, and 8 a crack formed in the layer 6.
The reason for the presence of these layers is as follows. Since the spatial intensity distribution of the high density beam 2 takes the form of a Gaussian-type body of revolution, that is, a cone-like body as shown in FIG. 2, thicker portions of the layer 6 are produced, and hence portions of the layer 7 are closer the position of the center of the beam 2 than others, causing the surface quality of the machine part 4 to be nonuniform, as shown in FIG. 3. In order to make such a surface uniform in quality, it is usual to irradiate the solidified layer 6 again with the beam 2 to re-melt and re-solidify it. In such a case, however, there may be further a crack or cracks 8 produced in the layer 6.
Further, since the positioning mechanism 5 of the conventional apparatus rotates the machine part 4 at a constant speed, the speed of the beam 2 relative to the portions of the surface of the machine part 4 which are not circular in cross section is not constant. Therefore, the thicknesses of the respective layers 6 and 7 become more nonuniform as shown in FIG. 4.
FIG. 5 is a graph showing the power distribution of the high density beam 2 over time. That is, the power of the beam 2 is substantially constant for a certain period with sharp leading and trailing edges. The beam power is substantially zero at the times when the beam is cut off. With this power distribution, a pond of molten metal is formed at a point on the surface of the machine part 4 which corresponds to the center of the beam at the time when it is cut off. This pond becomes a pit 9 after solidification, as shown in FIG. 6, which is a plan view of the machine part 4 in FIG. 3. In order to remove such pits, it is necessary to perform re-melting by irradiating the part 4 with the beam. However, it is undesirable to do so for the reasons mentioned above.
Accordingly, it is substantially impossible to improve the surface quality of the machine part 4 without the formation of pits and/or cracks, resulting in poor mechanical characteristics, including low strength and durability.